Disgusting service from Nashua Mobile & MTN!!!

I've had an MTN contract with Nashua Mobile which was renewed three times. When I travelled abroad, I went to the MTN store to block my line and for roaming. When I returned, I received a billing for something like R6000.00. I went to enquire at the Norwood branch where I asked the lady to block my line, only to find that it has been closed down. Now I keep getting harassed by different companies for something I'm not even liable for and every time I go to MTN, I get told that they cannot help me. In two years, I couldn't find anyone to assist to clear my name from this account.